add a dependabot config to automate vendoring

While dependabot has turned out great to automate updating dependencies,
a major painpoint was that we had to manually run `make vendor` for each
and every commit.  It was causing noise.

Adding the config file to `.github/dependabot.yml` will take of also
updating the `./vendor` tree.  `containers/common` is using this config
for a while successfully.

[NO TESTS NEEDED]

Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

.github/dependabot.yml

@@ -0,0 +1,7 @@
+version: 2
+updates:
+- package-ecosystem: gomod
+  directory: "/"
+  schedule:
+    interval: daily
+  open-pull-requests-limit: 10